Subsurface resistivity studies are often used to obtain information about subsurface geology, being considered inexpensive, fast, and non-invasive. Such studies are based on Ohm's law relating voltage (V), current (I), and electrical resistance (R) as V=IR. That is, for a given volume of material, a voltage applied across that material will result in a current flow through that material in proportion to its electrical resistance (or more generally, its electrical impedance).
Resistivity, typically denoted by the Greek letter “ρ,” is a material constant that is related to electrical resistance. It is more particularly a normalized form of electrical resistance that takes account of the cross-sectional area (A) of the volume—the current flowing perpendicular to this area, and the resistance decreasing in direct proportion to this area—, and the length (L) of the volume—the current flowing parallel to this length, and the resistance increasing in direct proportion to this length (ρ=RA/L).
Subsurface resistivity is typically measured by staking four electrodes into the earth, spaced apart from each other, using a current source to force a known current to flow through the earth between two of the electrodes, and measuring the voltage difference across the other two electrodes. The value of V/I is proportional to the resistivity (V/I=ρ L/A), and the constant of proportionality (L/A) is determined by a complex computerized calculation.
While such resistivity studies are relatively inexpensive and fast compared to known alternatives, they still require visiting the site and using specialized equipment to make measurements. And while thought of as being non-invasive, such resistivity studies do require staking electrodes into the ground. Accordingly, there is a need for a method for determining geological subsurface resistivity that further reduces cost and increases speed by eliminating the need to visit the site or disturb the surface.